Groovyのちょっとしたこと「Grails3のAsyncを試してみる」 ref: http://qiita.com/aya_eiya/items/24ee197fbd38290c125e

AsyncController

package jp.eiya.aya.grails.sample

class AsyncController {
    def fooService
    def index() {
      log.info 'start index'
      fooService.sayFoo(Hero.get(params.id?:1))
      log.info 'end index'
      return 'foo'
    }
}

BootStrap.groovy

import footest.*

class BootStrap {
    def fooService

    def init = { servletContext ->
      def clz = new Clazz(id:1,name:'archer').save(flush:true)
      new Hero(id:1,name:'Shiro Emiya',masterName:'Rin',clazz:clz).save(flush:true)
      clz = new Clazz(id:2,name:'saber').save(flush:true)
      new Hero(id:2,name:'Artoria Pendragon',masterName:'Shiro',clazz:clz).save(flush:true)

      fooService.changeMaster()
    }
    def destroy = {
    }
}

Clazz.groovy

package jp.eiya.aya.grails.sample

class Clazz {
    Long id
    String name
    String toString(){"$name($id)"}
    static constraints = {
    }
}

file6.txt

org.hibernate.LazyInitializationException: could not initialize proxy - no Session

file7.groovy

    task {
      log.info 'start task foo'
      Thread.currentThread().sleep(5000L)
      try{
        log.info ((h.clazz.name=='saber')?"Is dinner ready yet $h.masterName?":"$h") // error by no session
      }catch(e){
        log.error 'error',e // <--------------- 出してるのココね。 
      }
      log.info 'end task foo'
    }

file9.groovy

package footest

import grails.transaction.Transactional
import static grails.async.Promises.*

@Transactional
class FooService{
  def changeMaster(){
    log.info 'start changeMaster'
    task {
      log.info 'start task changeMaster'
      Hero.withNewSession{
        def sclz = Clazz.findByName('saber')
        def saber = Hero.findByClazz(sclz)
        ['Caster','Rin'].each{mstr->
          Hero.withTransaction{
            Thread.currentThread().sleep(15000L)
            try{
              saber.masterName = mstr
              saber.save(flush:true)
              log.info "saber's master changed to $mstr"
            }catch(e){
              log.error 'error',e
            }
          }
        }
      }
      log.info 'end task changeMaster'
    }
    log.info 'end changeMaster'
  }

  def sayFoo(Hero h){
    log.info 'start sayFoo'
    h.discard()
    task {
      log.info 'start task foo'
      Hero.withNewTransaction{
        h.attach()
        Thread.currentThread().sleep(5000L)
        try{
          log.info ((h.clazz.name=='saber')?"Is dinner ready yet $h.masterName?":"I'm here. $h.clazz") // error session failed
        }catch(e){
          log.error 'error',e
        }
        log.info 'end task foo'
      }
      log.info 'end sayFoo'
    }
  }
}

FooService.groovy

package jp.eiya.aya.grails.sample

import grails.transaction.Transactional
import static grails.async.Promises.*

@Transactional
class FooService{
  def sayFoo(Hero h){
    log.info 'start sayFoo'
    h.discard() // dettach from main thread's session
    task {
      log.info 'start task foo'
      Hero.withNewSession{  // use withNewTransaction in case you need transaction.
        h.attach() // attach with async thread's new session
        Thread.currentThread().sleep(5000L)
        try{
          log.info ((h.clazz.name=='saber')?"Is dinner ready yet $h.masterName?":"I'm here. $h.clazz") // now OK
        }catch(e){
          log.error 'error',e
        }
        log.info 'end task foo'
      }
      log.info 'end sayFoo'
    }
  }
}

Hero.groovy

package jp.eiya.aya.grails.sample

class Hero {
    Long id
    String name
    String masterName
    Clazz clazz
    String toString(){"$masterName x $clazz[$name]"}
    static constraints = {
    }
}